Thermal storage offers significant potential for coping more economically with cyclic heating and cooling loads in buildings. Thermal storage concepts are intimately associated with the energy conservation field and with today's energy problems, consideration of the advantages to be gained from thermal storage, properly implemented, is essential.
Heating storage can be effected by storing surplus heat from an occupied period in a building for reuse during an unoccupied interval. Where fuel rates for generated heat are higher than energy rates for reclaimed heat, storage can reduce heating costs. Those skilled in the art to which this invention relates will appreciate that a typical heat gain-loss chart for a building having heat reclaim and a changeover point of about 10.degree. F. illustrates the immense amount of heat surplus in a building every year compared with the amount of heat required to be generated. With an appropriately designed thermal storage system in a building having a 10.degree. F. changeover, there could be upwards of 67% of the generated heat requirement provided out of the thermal cushion for a 50.degree. F. differential. A significant saving in fuel costs may be made.
Cooling storage permits the use of smaller chillers, which can regenerate storage during unoccupied intervals of a building and derive help from it for occupied hours. This does nothing to reduce daily requirements of the cooling load but it does reduce chiller device. If one took a typical chiller demand curve (chiller demand-% vs. time of day) and straightened it out over 24 hours, one would find that a chiller machine of less than 50% the size required on the typical office building load and going flat out would develop about the same ton/hours as the typical machine. The smaller machine demands less electricity at any one period of time, and for demand-sensitive electric rates, the seasonal cost of cooling energy can be reduced significantly. For example, in Ontario, a demand reduction in electricity of 30% in a typical community would provide about a 20% saving in the power bill for electric cooling. The saving would be even greater, i.e., about 32%, in Toronto. At least 90% of the communities in Canada are demand sensitive with regard to electricity costs.
Buildings themselves provide their own storage which can be used advantageously if the control system is designed for that purpose. For example, in cooling seasons, the building can be used to reduce cooling demand if the mass is precooled over-night and the temperature allowed to rise through acceptable limits during occupied hours. Building storage varies, but cooling demand can be reduced by up to 20% if average space temperature in the building is allowed to rise by 1/2.degree. F. per hour through the occupied periods of the day. Building mass is also available to reduce heating cost through use of solar gain during the day.
The use of water storage tanks properly incorporated with a heating and cooling system for a building provide an even more effective means of conserving energy through thermal storage. Such systems, in concept, store water in tanks at preselected temperatures, which water is drawn out of storage during occupied periods of the building to supplement the demands of the system at that time, with return water of the system being pumped back into the storage tank. During unoccupied periods of the building, the system continues to run primarily for the purpose of returning the stored return water to the preselected temperature for the next day cycle. The cost of storage tanks does not have to be an extra cost. There is a trade off in being able to purchase a smaller less expensive chiller. Furthermore, with demand-sensitive electricity rates and the potential saving in fuel, the use of thermal storage can pay for itself over a relatively short period of time.
However, in the past, such systems have not met with much practical success. One of the primary problems has been the temperature blending of water in the storage tanks, and although there may be cases where blending exacts no penalty (or is even desirable), there are many situations where blending can nullify the benefits of thermal storage. For example, consider the case of storage used to provide 42.degree. F. water for a daily cooling cycle. Blending from returning 60.degree. F. water, if permitted, would preclude the latent value of the chilled water long before the sensible cooling effect of the storage was exhausted. Systems which have depended on the principle of buoyancy for anti-blending have been unsuccessful and this is particularly true of chilled water in the 40.degree. F. to 60.degree. F. range where the buoyancy effect of water is at its least. Another problem is encountered in using a thermal storage system wherein the storage tanks are at the bottom or are bottomside of a multi-storey building. The pressure in a chilled water line, for example at the top of the building, may be about 35 psi, whereas at the bottom of the circuit, the pressure due to static regain may be about 150 psi. The pressure in the storage tank circuit may only be about 30 psi or lower. It is possible to separate the hydraulic head of the building from the open storage through the use of a convertor. However, such convertors are not only massive, but are expensive as well. Furthermore, the convertors are up a vital 5.degree. F. of a narrow cooling storage range.